The document, R. Pettigrew, “Analysis of Grating Imaging and its Application to Displacement Metrology,” appearing in SPIE Vol. 36, 1st European Congress on Optics Applied to Metrology (1977), pages 325 to 333, describes a position-measuring device that is based on an optical 3-grating scanning principle. Such a position-measuring device makes it possible to generate phase-shifted incremental signals with regard to the relative movement of two objects that are displaceable relative to each other in at least one measuring direction. One of the two objects is connected to a measuring standard that extends in the measuring direction and includes at least one scale grating. In the case of a transmitted-light scanning principle, the scale grating includes scale-grating regions of different optical transmittances that are periodically disposed in the measuring direction at a first periodicity d1. A scanning unit that includes the scanning-side components of the position-measuring device is connected to the other object. Among these components are at least one light source, a scanning grating, and a detector system. The scanning grating includes scanning-grating regions that are periodically disposed in the measuring direction at periodicity d2 and feature different optical properties. The detector device includes a plurality of detector regions that are sensitive to radiation and are periodically disposed in a detection plane in the measuring direction at a third periodicity d3. The bundles of rays emitted by the light source first impinge upon the scale grating and subsequently pass through the scanning grating. The interaction of the bundles of beams with the scale grating and the scanning grating produces a periodic fringe pattern having a third periodicity d3 in the detection plane of the detector system. By scanning the fringe pattern or the periodically disposed detector regions with the aid of the detector system, it is possible to generate a plurality of phase-shifted incremental signals.
Such position-measuring devices, for example, are used in applications in which the position of a displaceable machine component in relation to a stationary machine component must be detected with the highest precision in a machine in order to carry out an exact relative positioning of these machine components via a machine control. If these machines involve machine tools, for instance, then this results in operating conditions that may adversely affect the operability of the optical position-measuring device. For example, it is possible that contamination such as cooling lubricants or oil mist are deposited on optical components of the position-measuring device, e.g., on the scanning grating. In the extreme case, this may cause the position-measuring device to fail.
To protect the scanning grating in a comparable scanning configuration of an optical position-measuring device, it is described in German Patent No. 32 10 614 to position the particular side of the carrier element on which the scanning grating is situated so that it faces away from the measuring standard. In addition, the scanning grating is coated by a protective element that is transparent to light. Since the measuring standard constitutes the second impinged-upon grating in this position-measuring device, contamination of the protective layer of the measuring standard and contamination in the interspace between the measuring standard and the third grating may also lead to especially disadvantageous adverse effects on the signals.